Agrobacterium tumefaciens causes tumors, called crown galls, by transferring to plant cells a small piece of DNA called T-DNA. In the bacterium, the T-DNA is part of a large virulence element called a Ti plasmid. This bacterium is the primary tool to transfer genes of agronomic benefit to plants of agricultural value including corn, soybean, cotton, rice, and wheat crops. Currently, derivatives of A. tumefaciens are used in which the T-DNA has been modified such that it lacks the tumor-inducing genes native to the system, and has added to it, the genes desired to be transfer to the plant.
Although A. tumefaciens strains are currently used, there are problems with the system. First, certain agriculturally-important plants are recalcitrant to gene introductions by the bacterium. Second other sets of plants can be transformed by A. tumefaciens, but the efficiencies are very low, making the development of transgenic derivatives of these plants an expensive and labor-intensive project.
Researchers have worked on improving the Agrobacterium system for transferring DNA to plants. Many studies have been directed towards developing conditions that improve the susceptibility of the plant to infection by Agrobacterium. Such approaches have proven useful in developing or improving transformation efficiencies in certain plants. From the perspective of the bacterium, most studies have focused on the Ti plasmid and on the genetic constructs, called binary plasmids, which carry the transgene. These two approaches have yielded limited successes. A third strategy has focused on mixing T-plasmids from one set of Agrobacterium isolates with the chromosomal background of other Agrobacterium isolates in an effort to identify combinations that are more effective than the natural pairings. With the exception of strain A281, and its derivatives EHA101 and EHA105, this approach has not been successful.
The development of a strain of A. tumefaciens that is significantly more efficient in transferring DNA to plant cells is thus needed for developing transformation systems for recalcitrant plant species and for improving the efficiency of transformation of plant species that are transformable at only low efficiencies.